Delignification of lignocellulose with an nu-(lower alkyl)-2-pyrrolidone



United States Patent "ice 3,272,689 DELTGNTFICATIION 0F LIGNOCELLULOSE WITH AN N-(LOWER ALKYL)-2-PYRROLIDONE Bernard L. lLenz, Stockholm, Sweden, assignor to Owens- Illlinois, Ina, a corporation of Ohio No Drawing. Filed Feb. 4, 1963, Ser. No. 256,131 8 Claims. (Cl. 162-72) The present invention relates to a process for the delignification of lignocellulose. More particularly, this invention relates to a process for delignification in which the pulping medium is a mixture of water and an N-(lower alkyl)-2-pyrrolidone.

The term lower alkyl is used herein to mean straight or branched chain saturated aliphatic radicals which contain from one to five carbon atoms; examples of the N-(lower alkyl)-2-pyrrolidones of this invention are N- methyl-, ethyl-, n-propyl-, isopropyl-, n-butyl-, secbutyl-, iso-butyl-, t-butyl-, 'n-pentyl, iso-pentyl-, and neopentyl, -2-pyrrolidone.

A hydrotrope is a substance which increases the solubility of another substance in water; the original definition (Neuberg, J. Chem. Soc, 110, II, 555) applied the term only to salts. The interaction between solute and hydrotropic agent which effects increased solubility is not thoroughly understood, but it is probable that the solute benefits from the superior solvation of the hydrotrope by bonding to it. The resultant bonds are usually readily broken by addition of solvent, which causes precipitation of solute, and it is therefore inferred that solute-hydrotrope interaction does not involve covalent forces, but depends on a combination of van der Waals forces, electrostatic attraction, and coordinate-covalent bonds.

The ability of certain hydrotropes to increase the water solubility of lignin more than they increase the solubility of cellulose is the basis of hydrotropic pulping. This method of delignification has advantages over procedures which require acidic (sulfite process) or alkaline (kraft and soda processes) reaction media (McKee, Ind. & Eng. Chem. (4), 38, 382 (1946)). An acidic medium usually causes hydrolysis of alpha-cellulose, and consequent decrease in yield of pulp; a further disadvantage is that the lignin is obtained as a highly modified calcium sulfonate. An alkaline medium usually causes extensive destruction of hemicellulose, and converts. the lignin to sodium salts of organic acids which possess mainly fuel value. The ability of hydrotropic processes to operate near neutral pH results in a superior yield of alpha-cellulose, and produces lignin which, because it more closely resembles native lignin, is a more promising material for further applications. Hydrotropic pulping usually involves a procedure in which lignocellulose, water and hydrotrope are allowed to react at an elevated temperature for several hours; the resultant pulp is removed by filtration, and water is added to the filtrate to precipitate lignin. Then lignin is separated by mechanical means, and the mother liquor is concentrated by evaporation or distillation of exess water; the resultant liquor is returned to the pulping step. This procedure is readily adapted to a batch or continuous process.

Accordingly, it is an object of this invention to pro vide a process for the delignification of lignocellulose.

It is a further object to provide a process for delignification in which the pulping agent is readily recovered and recyclized.

It is a still further object to provide a process for delignification in which the pulp and lignin are readily separated in substantially unaltered form.

It is another object to provide a process for delignification in which the pH is near 7.

Other objects and advantages of the present invention will be apparent from the following description.

3,272,689 Patented Sept. 13, 1966 According to the present invention, these and other objects and advantages are met by a process which comprises contacting lignocellulose with a reagent in which the essential ingredients are water and an N-(lower alkyl)- 2-pyrrolid-one to effect delignification. I

In the procedure of the present invention, lignocellulose, for example wood chips, is added to a mixture of water and an N (lower alkyl) 2 pyrrolidone. The weight ratio of these reagents is usually 1:20 to 1:1, respectively. The resultant mixture is heated usually at C. to 200 C. for a time of from about one-half to about 24 hours. The cellulosic material is removed from the mixture thus obtained by mechanical means, such as by filtration or centrifugation, and the mother liquid is treated with water to precipitate lignin and associated materials. The precipitate is removed, and the remaining liquid is concentrated by distillation; the resultant concentrate may be re-used in the pulping step. A particular embodiment of this invention involves treatment of wood chips with a mixture of water and an N-(lower al-kyl)-2-pyrrolidone in a weight ratio of from 1:4 to 1:2 at C. to 180 C. for a time of from one to four hours, followed by filtration to remove pulp, addition of water to the filtrate, removal by filtration of the lignin thus precipitated, and concentration of the aqueous N-(lower alkyl)-2-pyrrolidone solution. A preferred embodiment of this invention involves treatment of hardwood chips with a mixture of water and N-rnethyl-Z-pyrr-olidone in a weight ratio of from 1:4 to 1:2 in a sealed reaction vessel at a temperature of 100 C. to 180 C. for a time of from one to four hours, followed by the separation and concentration steps cited above.

The process of this invention is applicable to lignocellulose generally including softwoods such as pine and cypresses; hardwoods such as black gum, aspen, oak and fruit woods; and fibers such as those of bagasse and bast. These materials can be used in any convenient form, chips or meal being preferred, and can be previously extracted with organic solvents such as ethanol-benzene to remove fatty acids, sterols and glycerine, without material effect on the pulping procedure of this invention. The hardwood pulp thus obtained is suitable for use as a corrugating medium, and softwood pulp prepared by this procedure is useful as liner for corrugated board.

In general, less time and/or lower temperatures are required for pulping of hardwood than for pulping of softwood by the procedure of this invention; even with more extensive treatment, softwood pulp thus produced contains a higher precentage of lignin. Hardwood or softwood which has not been extracted will donate fatty acids to the pulping medium; these can precipitate with the lignin or distil during concentration. The presence of fatty acids causes the pH of the solution, originally in the range 9-11, to be reduced. The amount of water required for delignification is low; wood which has not been dried before pulping can contain enough moisture to support the pulping, so that no water need by added with the hydrotrope. Water from this source is included in the cited ratios.

A. In a typical embodiment of this invention a mixture of 30 grams of black gum wood meal, 280 ml. of N-methyl-Z-pyrrolidone and 20 ml. of water was stirred and heated in an autoclave at C. under a nitrogen pressure of 100 p.s.i.g. for three hours. The reaction mixture thus obtained is allowed to cool to room temperature, and then the pulp is removed by filtration; the yield is 69%, based on the weight of wood. The filtrate may be re-used two or three times without further treatment. When the pulp yield drops substantially, the filtrate is diluted with 500 ml. of water, and the precipitated lignin and associated material are removed by filtration;

about 26% of the lignocellulose appears in this fraction. The mother liquor is concentrated by distillation of excess water and the concentrate thus obtained may be reused as a pulping medium.

B. By the procedure of part A, 30 grams of cypress chips are treated with a mixture of 120 m1. of N-propyl- 2-pyrrolidone and 80 ml. of water in a stirred autoclave at 185 C. for ten hours. The resultant pulp and liquor are subjected to the separation procedures described above.

C. By the procedure of part A, 30 grams of black gum wood meal is treated with a mixture of 140 ml. of N-(Z-butyl)-2-pyrrolidone and 60 ml. of water in a stirred autoclave at 100 C. for 1 /2 hours. The resultant pulp and liquor are subjected to the separation procedure described above.

I claim:

1. A method of delignification which comprises treating wood with a reagent which comprises water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:20 to 1:1 at a temperature between 80 C. and 200 C. for a time of about /2 hour to 24 hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

2. A method of delignification which comprises treating wood with a reagent which comprises water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:4 to 1:2 at a temperature between 100 C. and 180 C. for a time of from one to four hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

3. A method of delignification which comprises treating hardwood with a reagent which comprises water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:20 to 1:1 at a temperature between 80 C. and 200 C. for a time of about /2 hour to 24 hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

4. A method of delignification which comprises treating hardwood with a reagent which comprises water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:4 to 1:2 at a temperature between 100 C. and 180 C. for a time of from one to four hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

5. A method of delignification which comprises treating softwood with a reagent which comprises water and N-methyl-Z-pyrrolidone in a weight ratio of from 1:20 to 1:1 at a temperature of from C. to 200 C. for a time of about /2 hour to 24 hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

6. A method of delignification which comprises treating hardwood with a reagent which comprises water and N-methyl-Z-pyrrolidone in a weight ratio of from 1:4 to 1:2 at a temperature of from C. to C. for a time of from one to four hours and separating a ligninenriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

7. A method of delignification which comprises treating bagasse with a reagent which comprises water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:20 to 1:1 at a temperature between 80 C. and 200 C. for a time of about /2 hour to 24 hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

8. A method of delignification which comprises treating softwood with a reagent which comprises Water and an N-(lower alkyl)-2-pyrrolidone in a weight ratio of from 1:4 to 1:2 at a temperature between 100 C. and 180 C. for a time of from 1 to 4 hours and separating a lignin-enriched phase containing said pyrrolidone from a cellulose-enriched pulp phase.

References Cited by the Examiner UNITED STATES PATENTS 2,945,863 7/1960 Buc 162-72 3,036,950 5/1962 Martin 162-168 3,081,219 3/1963 Drennen 162-72 DONALL H. SYLVESTER, Primary Examiner.

HOWARD R. CAINE, Assistant Examiner. 

1. A METHOD OF DELIGNIFICATIN WHICH COMPRISES TREATING WOOD WITH A REAGENT WHICH COMPRISES WATER AND AN N-(LOWER ALKYL)-2-PYRROLIDONE IN A WEIGHT RATIO OF FROM 1:20 TO 1:1 AT A TEMPERATURE BETWEEN 80*C. AND 200*C. FOR A TIME OF ABOUT 1/2 HOUR TO 24 HOURS AND SEPARATING A LIGNIN-ENRICHED PHASE CONTAINING SAID PYRROLIDONE FROM A CELLULOSE-ENRICHED PULP PHASE. 